Returns an identifier for the mod object.
It is used to gather mods together when calling covers on them.

covers $mod

Returns true if and only if the effects of the mod $mod are already ensured by the current mod object,
in which case no actual TikZ code will be emitted for $mod.
Both mod objects are guaranteed to have the same tag.

declare $formatter

Returns an array reference of TikZ code lines required to declare this mod before using it,
formatted by the LaTeX::TikZ::Formatter object $formatter ; or undef if no declarations are needed for this mod.

apply $formatter

Returns the TikZ code that activates the current mod as a string formatted by the LaTeX::TikZ::Formatter object $formatter.